This invention relates generally to a switching device, and, more particularly, to a circuit breaker having an insulative housing.
Circuit breakers are one of a variety of switching devices, such as overcurrent protective devices, used for electrical circuit protection and isolation. A basic function of a circuit breaker may be to provide electrical system protection when an electrical abnormality (such as a short-circuit or other fault condition) occurs in any part of the circuit. In such devices, a moveable electrical contact is typically selectively engageable with a corresponding stationary contact to enable current flow. When a fault condition is detected, the circuit breaker moveable contact is automatically disengaged from the stationary contact to thereby disable current flow through the circuit. Such separating of the contacts may typically result in an arcing condition or event within the circuit breaker. To ensure the electrically live parts such as the separable contacts remain insulated, and to protect users from any arcing conditions, circuit breakers are typically constructed using a molded plastic housing or cassette enclosing the separable contacts.
In a typical rotary contact circuit breaker, electrical current enters the system from a power line. A conductive line-side strap and a conductive load-side strap typically protrude from the circuit breaker housing to facilitate connection with conductors in the electrical circuit. The current passes through the conductive line-side contact strap to a stationary contact fixed on a portion of the conductive strap within the circuit breaker housing, and then to a moveable contact. The moveable contact is fixedly attached to a conductive arm, and the arm is mounted to a rotor that is rotatably mounted in a cassette-type housing. As long as the fixed contact is engaged with, or in physical contact with, the moveable contact, the current passes from the fixed contacts to the moveable contacts to a portion of the conductive load-side contact strap disposed within the circuit breaker housing, and out of the circuit breaker via conductive cable coupled to the load-side contact strap and then to downline electrical devices or loads.
Typically, the circuit breaker housing, or cassette, is formed of two insulative mating housing portions rigidly coupled together with mechanical fasteners such as rivets. When coupled together, the two housing portions, or half-pieces, may define a seam along their corresponding mating surfaces, and further define at least one interior cavity between the mating housing portions or cassette half-pieces for housing the circuit breaker conductive parts, mechanisms, and arc chute assemblies. Additionally openings in the housing may allow the line-side and load side straps to protrude from the housing to enable connection to external circuit wiring.
In some instances, such as multi-pole circuit breakers of the type used in a three-phase electrical system, several conventional single-pole circuit breaker devices may each be housed in an individual cassette, and these cassettes, may be further cooperatively enclosed in a single conventional multi-pole circuit breaker housing.
In the event of an overcurrent condition (e.g. a short circuit), extremely high electro-magnetic forces are generated. These electro-magnetic forces repel the movable contact away from the stationary contact. In other cases, a tripping mechanism disposed within the circuit breaker housing acts to drive the movable contact away from the stationary contact. For example, when the moveable contact is fixedly attached to a rotatable arm, and the moveable contact is in contact with the stationary contact, it defines an “ON” condition for the circuit breaker. When the rotatable arm is pivoted to physically separate the stationary and moveable contacts, the circuit breaker is thereby switched to the “tripped” or “off” condition. When the circuit breaker contacts are rapidly opened or tripped, for example due to a detected short-circuit event, an electrical arc is produced between the contacts. Accordingly, there occurs a voltage corresponding to the source voltage between the stationary contact and the moveable contact, thereby carrying out the circuit breaker operation. It is common practice to employ an arc chute assembly to help extinguish this resultant arc.
Additionally, during such an arcing event, high-temperature ionized gasses are generated due to the arc, with resultant high pressure forces likewise being developed within the housing interior cavity. The ionized gas temperatures can reach or exceed 20,000° C. for several milliseconds, which can vaporize the conductors and adjacent equipment. Moreover, an arc flash can release significant energy in the form of heat, intense light, pressure waves, and/or sound waves.
Such ionized gases may conventionally be intentionally discharged through specific exhaust vent openings arranged in the circuit breaker housing. However, if the ionized gases are unintentionally discharged from even the smallest of openings, such as along a seam or other small openings in the housing around the line-side contact strap and load-side contact strap, the gases could transfer to an adjacent circuit breaker, or to nearby bus bar conductors, resulting in a phase-to-phase electrical fault. The expelled ionized gases could also cause a phase-to-ground failure with a grounded metallic panelboard enclosure within which the circuit breaker is mounted.
The circuit breaker housing must therefore be robustly coupled together to safely withstand the high pressures generated during an arcing event. Typically, a strong and relatively expensive molding material such as sheet molding compound (SMC) is used to form the housing, which requires a relatively expensive compression molding process.
It is also important that the corresponding housing pieces be mated together tightly along the seam to minimize an egress of the conductive gasses therethrough to prevent injury to nearby personnel or equipment. Since conventional circuit breaker housing portions are fastened together using strong mechanical fasteners such as rivets, undesired localized stresses in the mating housing portions may form, and thus s thickening or otherwise strengthening of the housing in the riveted areas is required. It would be advantageous to provide a strong circuit breaker housing with a robust, sealed seam formed using less expensive materials and rigidly joined together with fewer, or without, the use of mechanical fasters.